There are books about this, such as Edward Yourdon's Death March.
I'm going to say some major things you could do differently, rather than "should", because "should" implies you've done something wrong, and that's really for you to decide. If you've not broken the law or hurt anyone, it's really up to you. So, arguably you should have reported them to the proper authorities for the working-time thing. Both as a legal matter, and an ethical matter of protecting others from what was done to you. Of course you don't want to be the whistle-blower who ruins the company and everyone loses their awful jobs, but the official position, embodied in your working-time laws, is that these awful jobs are literally worse than no job at all. You may or may not agree now that you're on the far side of one such job.
As for "coulds":
Like everyone says, you could have declined the absurd hours to begin with. You take the job, they tell you to work 70 hours in week 1, and that everyone else is doing it. You say "no thanks, if everyone else was jumping off a cliff I wouldn't do that", and walk back out the door. They might be a bit annoyed that their hiring process got you to that point without mentioning the hours, but you don't feel guilty, and they've barely met you so they have other people to hate more.
You could have raised the show-stopping issues with your line manager as they came up. The PMs need not be in a position to tell a lie to your line manager that you've only been telling them their project is absurd for 3 weeks, when in fact it has been several months. You could have told your line manager that their project was absurd at the same time you told them. Then, your line manager would have recognised them for pitiful blame-avoidant liars as soon as they said it.
This doesn't mean you have to CC everything to your line manager, but if you're facing unreasonable demands then it is part of your line manager's job to deal with the PMs to help manage your goals, your workload, and what counts as "success" for you in your job. If your job is impossible then it's your line manager's job to make it possible. Your line manager might also have a general interest in the company's projects succeeding. That doesn't mean your line manager would be able to deal with this bad project management, since the whole company sounds awful. But you could have given it a shot on the grounds that it's how things are "supposed" to work, and professionally speaking there's something to be said for giving everyone a chance to do what they're supposed to.
If you think your line manager would have ignored all of this, or agreed with the PMs all along, then in a sense it doesn't make a whole lot of difference that they are lying now, because your ex-line manager is useless to you anyway. So, you can stop worrying about the lying ;-)
If you respect your your line manager but think they were hampered by ignorance, then you could even have given them the evidence of the lies at your point of exit. Won't help solve the problems, necessarily, but at least that one line manager now has a better idea what they're dealing with, and what their other reports are still facing. It may be too late for you to do that now -- perhaps you don't have access to all that email any more.
Both of these points about informing your line manager might also apply to informing the level above these PMs. But, reporting to your line manager what you're doing is always expected, whereas jumping over someone's head to report them to their boss is kind of an aggressive move even when you're in the right.
As a professional, regardless of the hours and the politics, you could have declined to work on a project you didn't believe in. But, you don't have to succeed all the time. It's OK to take a big risk on something that will probably fail, provided that whoever is paying for it consents. You took a shot, it didn't work out. So, believing that something will succeed is nice-to-have but it's an unreasonably high bar to apply as an absolute rule before working on a difficult project.
Similarly, as a professional, you could have declined to work on a project that was managed according to principles you didn't believe in: unreasonable workloads; refusal to listen to the concerns of the technical experts (you) or to acknowledge your best appraisal of the risk; refusal to address massive scope creep. Learn from this experience to identify the difference between "difficult but it might work", and "even if this somehow starts to succeed, poor organisation will just drag it back into failure".
What you could not do, is fix the mistakes of a group of PMs who have all agreed amongst themselves a bad way of doing things, and are determined to do them that way. So there's no need to feel guilty about not having done that.